This invention relates generally to engines having pistons reciprocating within cylinders, and particularly it relates to an improved piston ring assembly between a piston and a cylinder wall. The invention is especially useful in a Stirling engine.
It is known to use a piston ring for sealing between a reciprocating piston and the wall of a cylinder in which the piston reciprocates. It is important for the piston ring to exhibit an effective sealing action, but not at the expense of any significant impairment of the engine operating efficiency because of frictional losses resulting from the action of the piston ring on the cylinder wall as the piston reciprocates.
In Stirling engines, it is known to use piston rings made of certain materials which can operate without lubrication, examples of which are certain compounds of TFE fluorocarbons, such as Rulon. These materials have high coefficiencies of friction, and therefore while they can contribute toward sealing effectiveness of unlubricated piston rings, they do so at the expense of creating significant mechanical losses as the pistons reciprocate in the cylinders.
The piston rings of the known art are not closed but rather slotted or cut radially so that they form an open circle. The slot gives rise to some leakage and thus impairs the sealing function of the piston ring to a certain degree.
Prior efforts to improve the sealing effectiveness of the piston ring have involved the elimination of the slot and hence the use of a closed piston ring. These efforts have not been successful because a closed piston ring is subjected to thermal effects which give rise to expansion and contraction of the ring which causes it, upon shut-down of the engine, to lose contact with the cylinder wall. Thus upon subsequent start-up of the engine, sealing cannot be established. Further attempts to provide the reestablishment of sealing upon start-up have always given rise to increased friction losses during operation.
The present invention relates to an improvement in a piston ring assembly, particularly for a Stirling engine, which permits the use of an unslotted, closed ring which provides satisfactory sealing during operation, yet the invention prevents the loss of sealing effectiveness at starting without increasing the frictional losses during running.
The invention comprises the further attribute of eliminating most of the frictional loss during running and is accomplished in part through an advantageous construction and arrangement of the cross sectional shape of the closed piston ring and of the piston ring groove within which the piston ring is disposed. The design of the ring and groove are such that during running the average pressures on the outward and the inward axial faces of the ring are approximately the same and thus the force acting radially outward on the ring, and hence the friction, is minimal. During starting, when the engine is cold, a radially outwardly directed force acts on the ring forcing it into contact with the cylinder wall, thus establishing sealing. This force is relieved when the engine has warmed up so that it does not give rise to frictional losses. For the attainment of such mode of operation a temperature responsive element is disposed in the piston ring groove for coaction with the piston ring. When the engine is being started, the temperature responsive element, which may be in the form of a bi-metal annulus, is effective to exert a radially outward force around the circumference of the piston ring. This causes a flat radially outwardly facing outer surface of the piston ring to be urged flat against the cylinder wall in face-to-face contact. This condition ensues as the engine warms up.
When a certain temperature is reached, the bi-metal element has circumferentially contracted to relax the radially outwardly directed force on the piston ring.
A seal is provided between the piston ring and the piston ring groove to prevent leakage between the piston ring and the piston itself, and thus to eliminate any net mass transfer between opposite sides of the piston.
In order to facilitate communication of the pressure conditions acting on opposite axial faces of the piston to the piston ring, one or more radial grooves may be provided in selected locations on the piston ring.
The disclosed embodiment of the improved piston ring assembly comprises an annular piston ring having what may be considered as an L-shaped cross section consisting of an axial leg and a radial leg. The piston ring groove has a similar L-shaped cross section. In the disclosed embodiment the temperature responsive means takes the form of a bi-metal ring which is disposed within the piston ring groove to be effective on the radial leg of the piston ring. Radial grooves provided on the axial faces of the piston ring or the piston ring groove permit pressures from opposite sides of the piston ring within the piston ring groove to balance the pressure force acting on the sealing face of the piston ring. In order to prevent leakage between the two pressures thus communicated, a circular seal is provided between the groove and the piston ring. This seal is located in its own groove which is open to the piston ring groove. In the disclosed embodiment, this circular seal is in sealing contact with the radial leg of the piston ring.
Other advantageous features of the invention will be seen in the ensuing description of a presently preferred embodiment of the invention. The disclosure includes several drawing figures wherein like components are identified by like reference numerals.